Lead dioxide is a compound having a metallic electric conductivity. Since lead has excellent durability, lead dioxide is, in particular, very stable at an anodic polarization in an acidic bath and, furthermore, can be relatively easily produced by an electrodeposition method, etc. Lead dioxide has been widely used, for example, as an industrial electrolytic anode for the production of explosives such as peroxides, perchlorates, etc.; raw materials for oxidizing agents; syntheses of organic compounds; water treatment; etc.
By utilizing these characteristics, block lead dioxide electrodes were practically used in the 1940's. The electrode being used was formed by cutting a pot-form iron having a lead dioxide layer formed on the inside surface thereof by electrodeposition. However, the production thereof was very troublesome, and the production yield was bad; further, such an electrode had a brittleness specific to ceramics, and the specific gravity thereof was about 9, which was larger than that of iron, whereby the electrode was difficult to handle. Hence, the usable ranges of the electrodes were limited.
However, since titanium having an excellent corrosive resistance to anodic polarization in an acidic solution has been commercially used since the 1950's, the cost of titanium has lowered, and titanium is now used more in the chemical industries. For example, a light-weight and durable lead dioxide electrode composed of the combination of titanium and lead dioxide has been produced, that is, an electrode composed of a titanium core having electrodeposited lead dioxide on the surface thereof. However, in the electrode, the interface between titanium as the core material and the lead dioxide layer was passivated by the strong oxidative power of lead dioxide, which sometimes resulted in making the passage of electric current impossible. Since electrically conductive titanium could not be used as the electrically conductive member, the lead dioxide layer itself was first used as the electrically conductive member. Thereafter, by spot-like welding platinum onto the surface of titanium to form an anchor, the electric conductivity was ensured.
Also, it became possible to obtain a good electric conductivity by applying a platinum plating to the whole surface of the titanium. However, this resulted in cracking the lead dioxide layer (and if a part of the lead dioxide layer was broken, platinum having a high activity to ordinary oxygen generation caused a reaction which peeled-off the lead dioxide layer).
The inventors previously solved the foregoing passivation problem by using semiconductive oxides of valve metals each having a different valent number. On the other hand, since the electrodeposition thickness of the lead dioxide layer on the surface of the core material was from 0.1 to 1 mm, which was thicker than the thickness of ordinary plating, the problem of peeling off the coating by an electrodeposition strain could not be avoided. However, the problem is being solved by laminating or mixing .alpha.-lead dioxide and .beta.-lead dioxide or by variously selecting other electrodepositing conditions. However, from the viewpoint of improving the corrosion resistance of lead dioxide, increasing the electrodeposition strain is desirable and, hence, corrosion resisting particles are dispersed in the .beta.-lead dioxide layer, as disclosed in, for example, U.S. Pat. No. 4,822,459.
The lead dioxide electrode developed through the developing steps described above is considered to be an almost completed technique for an ordinary electrolytic reaction but it was experienced that when the lead dioxide electrode was used in a fluoride-containing electrolyte containing fluorine ions or fluoride ions for a long period of time, hair cracks formed even though they were very slight and the electrolyte permeated through the cracks into the titanium portion of the ground, whereby corrosion resisting titanium was dissolved out.
As a countermeasure for the fluoride-containing electrolyte, it has been proposed that iron is used as the core material in place of titanium, an intermediate coating is strongly applied thereto, and a lead dioxide layer is formed on the surface thereof to constitute an electrode. However, once cracks form in such an electrode, the electrode is not sufficiently satisfactory since the corrosion resistance of iron as the core material is far inferior to that of titanium.
As described above, various investigations have been made on lead dioxide electrodes and various solving methods have been proposed. However, a lead dioxide electrode having a sufficient corrosion resistance and practical use to a fluoride-containing electrolyte, which is frequently used and is considered to be increasingly used hereafter, has not yet been realized.